This invention relates to homogenizing viscous liquid material, particularly molten glass, by means of a plurality of stirrers. More specifically, the stirring arrangement of the present invention provides more effective homogenization than prior art arrangements.
Homogenization is important in the manufacture of glass because regions of very slight compositional differences can exhibit different indices of refraction from the bulk of the glass, with the result that the differential bending of light rays is visible as distortion of an image viewed through the glass product. Minimizing this effect is desirable for producing glass of high optical quality. The inhomogeneities may be present in the molten glass due to delayed melting of some raw materials or due to erosion of refractory walls with which it is in contact during the melting process. Some glasses develop inhomogeneity by vaporization of volatile constituents from surface portions of the molten mass in the melting furnace. Homogenization of molten glass is also important in those glassmaking processes in which colorants or other substances are added to the glass after melting has been substantially completed.
Various rotated stirrers including spiral types and paddle types have been used to mix and thereby improve the homogeneity of molten glass. A typical arrangement in larger scale, continuous glass producing operations includes one or more rows of stirrers extending across the path of molten glass travel along a channel or through a restricted waist structure. Examples of that type may be seen in U.S. Pat. Nos. 4,047,918 (Heithoff), 4,339,261 (Walton et al.), and 4,744,809 (Pecoraro et al.). Many particular stirrer structures have been used in the prior art and have potential applicability to the present invention. An example of a stirrer structure that has a particularly powerful stirring action is shown in U.S. Pat. No. 4,493,557 (Nayak et al.).
Most stirring arrangements have as their object to divert portions of the molten glass vertically and/or laterally from the overall flow path, thereby intermixing portions of the glass stream. However, it has been found that when a relatively wide row of stirrers is involved (i.e., four or more stirrers in the row), even stirrers having very powerful stirring action tend to have a localized effect such that distinct bands of glass downstream from the stirring zone are produced within which the glass is relatively well mixed but between which insufficient mixing has occurred. Increasing the stirring action seems to have little effect on the existence of these bands. The presence of these bands indicates the possibility that significant inhomogeneity remains in the molten glass which may deteriorate the optical quality of the product glass. It would be desirable to reduce this banding effect so as to assure better homogeneity of the glass.